The 6R7G double diode triode was designed for radio receiver use. In the classic superhet broadcast receiver such a valve would provide signal and AGC rectification, followed by the first audio amplification.
The 6SR7 has the same rating but is in a single ended metal envelope.
The triode is mounted above the double diodes on a common cathode.
A side view of the triode anode. The shaped mica sheets hold the assembly into the reduced width section of the envelope.
The classic envelope is 38 mm in diameter, and excluding the IO base pins, is 102 mm tall.
References: Datasheet, 1040 & 1043. Type 6R7G was first introduced in 1936.